In The Witcher Yaevinn you could is say is seen as a terrorist leader. This is due to Yaevinn being a part of the the Scoia'tael which many humans believe is a group only dedicated to due harm to the land of Temeria. Yaevinn in fact is a Scoia'tael commando that leads his people to fight the lack of equality for the Elder races. Although Yaevinn did not always believe this because at one time he just wanted to live out a normal life but eventually the discrimination against his people led him to join the cause of the Scoia'tael. It is also known Yaevinn once fought in second Nilfgaard War as a Lieutenant Colonel of the Deithwen division.
Yaevinn in The Witcher is the main representative of the Scoia'tael for Geralt. Throughout the story Geralt runs into Yaevinn and can often make the choice to side with him, Order of Flaming Rose, or remain neutral. These choices Geralt makes leads to many conflicts that shape the end of The Witcher.
Worth it's Weight in Gold
In chapter II Geralt for the first time meets Yaevinn of the Scoia'tael at the druid's grove in the swamp of Vizima in this optional mission. Yaevinn then asks Geralt to deliver a message to Golan Vivaldi and to return Vivaldi's reply. When returning to Yaevinn with Vivaldi's reply Geralt finds Yaevinn is no longer in Druid's grove but is rather in the north-east part of swamp called the non-human camp. In this camp Yaevinn is preparing to go to battle with Order of the Flaming Rose at the Golem Burial ground. Upon delivering the letter Yaevinn will tell Geralt some information on Berengar's situation.
In chapter II Geralt has to make a choice to not do this task, side with army of Order of Flaming Rose, or side with thearmy of Scoia'tael. By remaining neutral Geralt does not lead one these army of two factions to Golem Burial grounds to fight the other one. If Geralt sides with Yaevinn and the Scoia'tael they will win the battle and later Scoia'tael leaders will approve of Geralt's help.
Echoes of Yesterday
In the beginning of chapter III Geralt meets Yaevinn in the Druid's grove again. Upon speaking to Geralt Yaevinn asks a task for Geralt to help him locate some of his missing units in the sewers of Vizima. When Geralt arrives to place were missing units are he meets Ren Grouver a dwarf who ran away from a skirmish of his unit with bruxa. Upon killing the Bruxa Yaevinn arrives in a teleporter and gives Geralt his reward.
In Chapter III Geralt leaves the banquet he went to and is confronted by a boy. This boy tells him of the that the Scoia'tael are ransacking Vivaldi's bank that is no longer owned by Golan Vivaldi. When arriving to bank Geralt has to choose to side with Order, Scoia'tael, or not choose side at all. If Geralt chooses to open up a conversation with Scoia'tael in bank he has option to speak to Yaevinn and do a mission to retrieve the rights back for Vivaldi's bank by going into a dungeon to fight kikimores. While for Order Geralt kills the elves. After completing this Yaevinn no matter what decision flees and may go to Vivaldi's house in hideout.
In the end of chapter III Geralt has the choice to ask the Scoia'tael to aid him in the coming battle against the Salamandra. By doing so Geralt must ask Yaevinn at Vivaldi house to help and Yaevinn will then proceed to New Narakort. When at New Narakort Geralt, Triss, Declan and Yaevinn are planning to attack the Salamandra. During invasion of Salamandra base Yaevinn informs Geralt of his situation and helps him fight in the base.
The Ashes of Vizima, Hope Burns Bright, Sweet Revenge, Frozen Reflections
These missions rely upon the side Geralt took and are related to a series of events that caused the Elder races in Vizima to fight the racism and start a rebellion in Vizima. In these missions Yaevinn or Siegfried can help Geralt get to the bottom of the Salamandra mystery. In Ashes of Vizima Geralt has the choice to kill Yaevinn because depending on Geralt's choices before Yaevinn may blame Geralt for death of Touvriel. However if Geralt helped Touvriel then this quest will go differently. If Geralt sided with Scoia'tael the mission Hope Burns Bright will take place in which Geralt helps Zoltan escort people to Hospital in Vizima. In Sweet Revenge if Geralt is sided with Yaevinn then Yaevinn will help Geralt get to solution of Salamandra mystery. In last mission Frozen Reflections Geralt may have to fight the spirit of Yaevinn if he sided with Order or went neutral. If Geralt went neutral he has to fight both the spirit of Siegfried and Yaevinn.